A rotational shaft having a worm is manufactured by a rolling process with rolling dies. Namely, a rod material is pressed by and between a pair of rolling dies, and the rolling dies are moved relative to and in parallel to each other, so that the rod material is rotated between the rolling dies, to finally form the worm on an outer surface of the rod material.
It is, however, known in the art that a bending portion is likely formed at an end of the worm portion (a charged-up portion of a worm tooth (an incomplete worm tooth)), or the worm shaft is likely bent, due to a large bending moment to be generated at an end portion of the rolling dies during the rolling process.
When the worm shaft having such a bending portion is used in a product, for example, for an electric rotating machine, it may cause abnormal vibrations or noises with its rotation.
Accordingly, an adjusting process is generally necessary to adjust such a bending of the worm shaft after the rolling process. And therefore, a manufacturing process is inevitably long and a higher manufacturing cost is unavoidable.
Japanese Patent Publication No. 2003-145242 discloses a method of manufacturing a worm shaft, in which an adjusting portion is formed next to a worm portion. The worm portion is formed by a rolling process with a pair of rolling dies, and the adjusting portion is reformed by another pair of reforming dies arranged next to the rolling dies, so that a possible bending of the worm shaft is reformed to become a straight shaft by the reforming dies at the same time during which the worm portion is formed by the rolling dies.
During the rolling process, a surplus material, which is surplus material for forming worm teeth, is likely to flow toward axial ends of the worm shaft. If such surplus material were not escaped from a worm forming portion to the axial ends of the worm shaft, it may cause a stress at the worm portion to finally result in the bending of the worm portion.
In the above mentioned technology disclosed in the above Japanese Patent Publication, the possible bending of the worm shaft is reformed by the reforming dies. It does not, however, suggest a method of manufacturing a worm shaft, without such an additional adjusting process, having a small bending at the worm portion, in view of flowing (escaping) the surplus material to the axial ends of the worm portion during the rolling process.